Inline thumbnail not showing to standard users

[JulienT]

I have a simple question but haven't found an answer in the forum.
I set pictures attached to show as thumbnail.
If I login as admin and go to a post with a picture, i can see the thumbnail.
If I login with a normal user and go to that same post, the picture is seen as file attachment, not thumbnail.
What did I do wrong?

Julien

[JulienT]

I attach here 2 pics to show what I mean.

One is when logged in as Admin.
The other one is when logged in as standard user.